Portable color and appearance measurement systems are used in a variety of industries and disciplines. For example, portable spectrophotometers, such as Datacolor 45G® are useful for inspection and control, among other uses, of painted surfaces, plastic parts, package prints, automotive parts and other manufactured goods. These devices are capable of producing measurement results that correlate better to visual assessment. For items with multiple visible components like home electronics and automobiles, achieving correct color analysis is complicated by the need to visually match large numbers of components, usually with different colors and finishes. Devices in the prior art are calibrated to work on specific materials or within specific operation ranges. This requires additional time and expense to recalibrate devices to make different measurements for different surfaces. As a result, consistent measurements are difficult to obtain in a timely manner.
Spectrophotometers are known and widely used in various technical disciplines. Commonly owned U.S. Published Patent Application No. 2009/031649 and U.S. patent application Ser. No. 13/327,072, which are each hereby incorporated by reference, describe 45/0 spectrophotometers with ring shaped light sources for use in measuring the color of a sample. Spectrophotometers are used for the purposes of measuring and calibrating various sample colors and hues. In these devices, light is reflected off the surface of a sample, measured and recorded with optical sensors. Optical sensors have improved fidelity and reliability over human observation; however, the precision of the current art devices is in need of improvement.
In a typical color measurement device, such as in Datacolor's 45G® spectrophotometer, a sample measurement channel and a reference measurement channel are usually provided. The reference channel is used to compensate any fluctuation of the illumination intensity. The illumination in prior art devices is set at a level wherein both the sample channel signal and reference channel signal are receiving the maximum amount of illumination without oversaturation of the sensors. Usually, prior art optical signal detectors have an optimal range for the signal strength related to the strength of the light captured. If the signal strength falls into that optimal range, the detector will have good signal-to-noise ratio (SNR) and not be saturated. In this context, saturation of the sensor occurs when the sensor has reached its maximum threshold for linear differentiation of intensity. As a result, all measurements above the saturation point must be compensated for sensor nonlinearity, at a possible cost of impaired accuracy. Since the reference channel typically sees a stable signal with small variation, the signal strength of the reference channel is usually set lower than the sample channel so that more illumination can be delivered onto the sample.
Once the illumination of the above-mentioned color measurement device is set, the device provides repeatable and accurate performance when measuring high reflective samples, such as white tile or other bright colors, because both sample channel and reference channel work in the optimal range and have high SNR. However, when measuring low reflectance samples such as black tile or other dark colors, the sample channel signal drops significantly, and thus the SNR in the sample channel will be low. Therefore, prior art devices have worse performance when measuring dark colors as compared to bright colors.
The deficiencies in the prior art render the ability to measure a sample difficult and inconsistent when the sample has low reflectance or dark colors. Therefore, what is needed in the art is a spectrophotometer that provides improved color measurement functions across the illumination spectrum. What is also needed in the art is such a system and apparatus that also allows for precise measurement of low reflectance surfaces. What is further needed is such a system and apparatus that is capable of dynamically adjusting multiple reflectance measurements to achieve accuracy and precision measurements across the wavelength range.